This invention relates to a new process for recovering energy from a compressed gas (such as compressed flue gas) which has been cooled and is then reheated using low-quality heat sources such as circulating water in a power plant and then expanded through a turbine (or other expansion engine), to recover the energy in the heated compressed gas stream.
The need for separation of carbon dioxide or other vapor constituents such as sulfur dioxide or water (in this invention the term “vapor” can represent any condensable gas) from a flue gas stream may involve compression of that flue gas stream and cooling of the stream to condense the vapor. Once a portion of the vapor has been removed through condensation, the remaining gas stream can have energy recovered from it by expansion through an expansion engine (such as a turbine). Presently, high-concentration carbon dioxide gas streams are treated using compression and cooling to obtain liquid and solid CO2 in industry. However, the expansion of the resulting waste gas stream, which is cold and depleted in CO2 content, is not used effectively for energy recovery and the use of low-quality heat sources to heat that gas stream for enhanced energy recovery is not applied.
There is now a growing interest in a method to remove CO2 and other vapors from the flue gas stream at power plants or other industrial combustion facilities (to slow the increase in concentration of atmospheric greenhouse gases and to remove other local pollutants) which will also increase the cost of gas-stream processing. The present invention recovers revenue in the form of otherwise lost energy in the cold compressed gas stream.
The energy recovered can help offset the extra energy required for the additional compression used for more complete separation of the vapors or if the components of the flue gas are required at high pressure such as in a process for mineral carbonate sequestration, injection into saline aquifers, reactions with brines, enhanced natural gas recovery, or other sequestration methods. This is different from the prior art as now practiced where the state of-the-art processes for compression separation of more dilute gases are significant net energy users, see our co-pending patent application entitled “Compression Stripping of Flue Gas with Energy Recovery”, filed Dec. 4, 2002, Ser. No. 10/309,251, by the inventors of this application the entire disclosure of which is incorporated by reference. GE-Enter Software's power plant modeling package “GateCycle” was used to model this invention as well as our co-pending application.